The present invention relates to a hydraulic pressure control system for an infinitely variable belt-drive automatic transmission for a vehicle.
A known control system for an infinitely variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid-operated servo device so as to vary the running diameter of the belt on the pulley in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to provide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output torque of the engine. Recently, a control system has been proposed in which the transmission ratio is controlled so as to meet the requirement of small fuel consumption. In such a system, the line pressure also should be controlled to improve the fuel consumption. However, the conventional system, which controls the line pressure in accordance with the transmission ratio and engine speed, can not properly control the line pressure.